a. Field of the Invention
The present invention relates to an apparatus and method for processing images, in particular for determining the degree of blockiness in coded images.
The present invention describes a method to measure the blocking artefact in a digitally compressed still image or video signal. When a video signal is to be stored or transmitted over a telecommunications network, it is compressed using an encoding algorithm such that the encoded signal requires less storage space and can be transmitted over a reduced network bandwidth. Still images are also typically compressed for storage or transmission. The process of compression can introduce visual distortions and reduce the quality of the signal. Block distortion (also known as blocking or blockiness) is caused by image compression. It is characterized by the appearance of an underlying block structure in the image. This block structure is a common feature to all DCT (discrete cosine transform)—based video and still image compression techniques. Technically, it is often caused by coarse quantization of the spatial frequency components during the encoding process. In practice, blockiness appears when high data compression ratios are used, for example in order to transmit video content using a low bandwidth connection. Blockiness is subjectively annoying and for analysis of a perceptual quality of a decoded video signal it is helpful to identify and measure the level of blockiness in an encoded/transmitted video/image.
The main visual degradation appearing in digitally compressed image or video is caused by the coarse quantization of the transform coefficients in the compression process. Most modern image compression algorithms use a two-dimensional (DCT) producing a series of transform coefficients, which are then quantized. The quantization is at the origin of the visual distortion known as blocking artefact (or blockiness). A coarser quantization (larger quantization step) will usually cause stronger blockiness. Because the compression algorithm independently applies the DCT transform to blocks of M×N pixels, the compressed image will exhibit vertical and horizontal boundaries at the edges of the DCT blocks. Usual values for M and N are M=N=8 in video codecs such as MPEG-2, H.261 and H263.
b. Related Art
‘Intrusive’ or ‘out-of-service’ metrics which require comparison of a decoded signal to a reference signal such as those described in United Kingdom Patent Application No GB2347811 “Measuring blockiness in decoded video images”, United States Patent Application No US2007071356 2007 “Method and apparatus for blocking artefact detection and measurement in block-coded video”, “A Multi-Metric Objective Picture-Quality Measurement Model for MPEG Video”, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 10, NO. 7, October 2000 are of only background interest to the present invention which is focussed towards ‘non-intrusive’ or ‘in-service’ metrics as it is desirable to be able to analyse the degree of blockiness in a received decoded signal without having to compare the decoded signal to the original transmitted signal.
Methods proposed in prior art rely mostly on identifying the block boundaries introduced by the block-based compression algorithm and measuring intensity differences at these block boundaries. One common drawback of prior-art methods for measuring blockiness is that the exact location of block boundaries must be known or assumed. Spatial shifts of the block structure with respect to the origin are assumed to be zero, which may not always be the case. Likewise, blocks not aligned in a certain grid fashion (e.g. 8×8 grid or 16×16 grid) will not be properly recognized.